The present invention relates generally to an apparatus and method for dispensing sheets. More particularly, the present invention relates to a system and method for removing individual sheets from a stack and feeding the sheets to processing equipment.
Devices for feeding sheets of material are known. For instance, in the box making industry, stacks of corrugated cardboard sheets are fed to a processing line for making boxes. In a box making plant, such subsequent equipment is typically provided for printing, cutting slots, and folding corrugated cardboard sheets which travel down the line. Devices are known for continually removing individual cardboard sheets from such a stack, and for feeding those sheets to a line of subsequent processing equipment.
Box-making equipment is generally configured to process sheets at a particular rate. Therefore, it is important that a sheet-feeding device feeds a supply of corrugated cardboard sheets at the same rate that the other equipment operates. Feeding sheets with imprecise timing can result in a malfunction of a subsequent box making step. Therefore, it is desirable to provide a sheet feeding apparatus that can continually remove sheets from a stack at a precise rate and for transferring the sheets to a manufacturing line in a precise sequence and speed.
U.S. Pat. No. 5,048,812 discloses a prior art sheet feeding apparatus. That device includes a holder for storing a horizontal stack of cardboard sheets. A gate is provided at a front side of the holder so that the lowermost sheet of the stack can fit between a bottom edge of the gate and a series of rollers located beneath the holder.
In the system described in that patent, a first series of rollers are provided which are driven at a variable velocity, capable of accelerating a sheet from a linear speed of zero to a speed corresponding to the required linear sheet speed of the subsequent box-processing equipment in an assembly line. A second series of rollers operates at a constant velocity corresponding to the required linear sheet speed and immediately follows the first series.
In the prior art device, a stack lowering mechanism is provided which has a plurality of cam-driven reciprocating lifting pads. The lifting pads are used for movably supporting the stack, which can be lowered to place the lowermost sheet into contact with the rollers of the first series. Upon contact with the rollers, the lowermost sheet is transported away from the stack. As the sheet is transported away from the stack, the lifting pads are sequentially actuated upward by the cams, to hold the remaining stack out of contact with the first series of rollers.
The sequential actuation of the lifting pads in the prior art device corresponds to the position of the leading edge of a sheet being removed. More particularly, the prior art sheet feeder, as the leading edge of a sheet contacts a first roller of the second series, the rearmost lifting pads are actuated upward into contact with the stack. Similarly, as the leading edge of the sheet contacts a second roller of the second series, the next rearmost lifting pads are actuated upward into contact with the stack, and so on.
A problem with the prior art sheet feeder lies in the fact that the lifting pads can be actuated upward into the travelling sheet. When this happens, the traveling sheet becomes rubbed between the lifting pads and weight of the remaining stack which is in the holder. This rubbing effect continues until the trailing edge of the removed sheet has been pulled clear of the forwardmost lifting pad, making the process less efficient and accurate. The described problem is particularly apparent when the sheets are long.
Therefore, a need exists for an improved sheet feeding apparatus with a stack lifting means which does not rub a sheet being pulled from a stack.